In many conventional applications, such as wireless local area networks (WLAN) or Long Term Evolution (LTE), signals are operating at low amplitudes, which have high peak-to-average ratios and which cause transmitter performance to suffer. This is in part due to the performance of conventional power amplifiers (PAs) at these peak-to-average ratios, where the power efficiency drops exponentially. This is illustrated in FIG. 1, where the performance of a class AB amplifier, a switching PA, and a Doherty PA are compared. As shown, Doherty PAs have the best performance, but these PAs are bulky (using large power combiners that are not suitable for CMOS processes) and are generally employed in base stations. Switching PAs (which can use polar or linear amplification with nonlinear components (LINC) architectures) have better performance than Class AB PAs, but there are several problems with these architectures as well (such as high sensitivity to delay mismatches and efficiency limitations due to the use of power combiners). As shown, none of these PAs meet the target performance.
There are also a litany of other problems associated with these architectures, and, to illustrate some of the problems associated with these conventional architectures, one may look to fully digital transmitter 100 shown in FIG. 2. In operation, the digital modulator 102 is able to generate in-phase (I) and quadrature (Q) signals for a modulator. In the modulator, the local oscillator (LO) 107 generates an LO signal that is phase shifted by the phase shifting circuit 106 (which is typically a hybrid) so as to provide a 0° phase shifted signal to mixer 104-1 and a 90° phase shifted signal to mixer 104-2. Mixers 104-1 and 104-2 are then able to mix the I and Q signals with the phase shifted LO signals, and the outputs of mixers 104-1 and 104-2 are combined with combiner 108 (which is typically an adder). The output from the modulator is then filtered by the pulse generator 110 (which can, for example, be a 1-bit band-pass sigma-delta modulator (SDM) or a 1-bit carrier pulse modulator) so as to generate signals for the switching PA 112. Some problem with this arrangement are that: (1) when an SDM is used as the pulse generator 110, the load generally includes bulky and expensive analog post-filtering and (2) varying pulse duration during one RF cycle to another can be severely limited.
Therefore, there is a need for an improved RF transmitter.
Some examples of conventional circuits are: U.S. Pat. No. 7,729,445; European Patent No. EP1632073; Midya et al., “Quadrature Integral Noise Shaping for Generation of Modulated RF Signals,” Proceedings of the 45th Midwest Symposium on Circuits and Systems, Vol. 2, pp. 537-540, 2006; and Wagh et al., “An all-digital universal RF transmitter,” Proc. IEEE Custom Integrated Circuits Conf. (CICC), p. 549, 2004.